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Aux Fan

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    Aux Fan

    I have a 1990 325i manual with air conditioning. There is an auxiliary fan in front of the radiator that I have never heard running. I have always assumed it was to come on when the ac was switched on but in testing my ac unit (still with the original R12)
    this weekend it did not. What controls this fan and how do I test to see if it is working correctly.

    Time to break out the wiring diagrams. First, does your AC work at all?

    At a high level, when you push the AC button, the AC vent sliders have to be in the correct position, then the Evaporator Temp Sensor need to be above freezing. From there, power goes to the fan, through the pressure switch on the dryer, then to the compressor.

    The fan has two circuits, a low speed and a high speed (when the engine gets too hot). The low speed has a resistor right in front of the fan. Check for 12 volts there if everything else is working. How does the fan feel if you spin it by hand?
    2004 525i Manual - 1985 325E Coupe Manual


      fastair, its best to keep the conversation in the thread versus private message so the next owner with a similar problem sees the entire story. So, you think the AC is working as the pressures appear correct and the fan can be turned by hand. All good.

      At the front of the fan, maybe you can get there with the kidney grills removed, there is a resistor (about a half inch square, and two inches long?). One wire on one side (black), two on the other (red).

      The black side controls the fan when you turn on the AC (low speed circuit). Check for 12 volts on that side. If that's good, check for voltage on the other side (red). Those resistors are know to go bad (crumble in my case).

      The red circuit will kick in on a high temperature situation and the fan will go high speed as the resistor is out of the circuit.

      Two things for everyone to keep in mind. If we keep the conversations in the thread, everyone sees the entire story. And close the thread with the end result so the next person with a similar problem sees the end result.

      "Experience not shared, is knowledge wasted"
      2004 525i Manual - 1985 325E Coupe Manual


        Tomstin, I cant get 12V to the black wire that turns on the condenser fan. I can jump the temp switch on the radiator to make my fan run, Ive installed a spal fan without the low speed resistor, so my fan runs on high whenever its engaged.

        Is it reasonable to assume that since my aux fan will not turn on with the air cond. turned on, that I am having a bad relay condition in the fuse box? Or, would it be the high pressure switch on my dryer? My dryer has both the low and high pressure switches.

        My a/c works fine, I just installed a new condenser and fan and new dryer, system is charged and pressures are looking ok. Just cant get the fan to work like it should.

        My son has the 1987 325e, 2 door, 5speed
        I daily the 1989 325i, 4 door, 5speed


          Take a look at the wiring diagram for your specific year. Looking at the 85 diagram, take a look at fuse 3 and the K1 relay feed that fan from the AC and are independent from the High Temp circuit.

          The fan seems to get its ground and will complete that circuit without the compressor / dryer.
          2004 525i Manual - 1985 325E Coupe Manual